A screw driver is used for driving screws to fix a plate member such as a plaster board to a ceiling or a wall. Laid open Japanese Patent Application Publication No. H5-318331 discloses such screw driver having a driving assembly including a motor, a driven assembly having an end bit mounting portion, and a clutch mechanism that performs frictional power transmission from the driving assembly to the driven assembly. No power transmission to the end bit is performed without generation of friction when the clutch mechanism is rendered OFF, and power transmission to the end bit is performed with the frictional force when the clutch mechanism is rendered ON.
According to the power tool disclosed in the Japanese publication, frictional force is generated at the clutch mechanism upon pressing the end bit against a screw. In this case, friction is slightly generated even after turning OFF the clutch mechanism, so that driving force is slightly transmitted to the end bit along with the forward rotation of the motor. Therefore, alignment of the end bit with a groove of a head of a screw becomes difficult to achieve. In order to avoid this problem, an O-ring is interposed between a housing and an end bit mounting portion. However degradation may occur in the O-ring due to long term use, and as a result, the O-ring cannot prevent the end bit from being rotated along with the forward rotation of the motor in spite of turning OFF the clutch mechanism.
Particularly, the end bit is reversely rotated by pulling a trigger after simply setting the end bit onto the groove of the head of the screw without sufficient pressure against the screw head in case of detachment of the screw from the workpiece. Conventionally, a switch is provided to change rotational direction of the motor between forward rotation and reverse rotation. If a user overlooks to change the switch to the reverse rotational position, but unintentionally maintains the forward rotational position of the switch, it would be difficult to align the end bit with the groove of the head of the screw, because the end bit may be unintentionally rotated in the forward direction even in the turning OFF state of the clutch mechanism.